The checking of the integrity of the structures (works of art, airplanes or pipe-lines for example) during their life is done generally during maintenance operations, with human inspection and intervention. Concrete technical problems consists for example in detecting and dimensioning a corroded zone on an airplane fuselage.
For these integrity checks, so-called non-destructive testing (NDT) methods are generally used, by so-called “conventional” methods (ultrasound, electromagnetic methods, etc.).
For a number of years now there have been research developments aiming to incorporate sensors in the structure at key points of the structures, and do so in order to automate the measurements (for example at regular intervals, these intervals being generally close together in time) and to be able to access information on the state of health for certain inaccessible zones, without dismantling or interrupting the operation of the structure. In general, these developments aim to space apart the maintenance intervals and therefore save money.
In particular, some research provides for the use of guided ultrasound waves (GW) emitted and detected by piezoelectric transducers (for example of PZT type) in-corporated in the structure. These guided waves are propagated over a great distance (some tens of cm to some hundreds of meters in highly favorable geometries such as pipelines), so that a limited number of transducers makes it possible to test a large area. Other technologies can be used to emit and/or detect the guided ultrasound waves (in addition to the optical fibers, PVDF films or magnetostrictive sensors for example).
One general technical problem lies in finding an acceptable trade-off between the number of sensors to be incorporated (cost, bulk, weight, etc.) and the quality of the information that can be recovered by these sensors. A high number of sensors means a high cost and a low number of sensors often means a lack of reliability of the information, risks of false alarms, or even a lack of redundancy in the event of failure of a sensor. The multiplication of the sensors however poses other specific problems (for example each incorporated sensor can constitute an additional point of embrittlement, which could risk inducing new defects in the structure). For each sensor, it is also essential to provide electrical power supply wires, which is not always possible. In industrial practice, very few applications provide a satisfactory trade-off.
Regarding the nature of the sensors, the solutions known from the prior art using lasers as measurement systems cannot be used in all circumstances. In particular, the lasers cannot be incorporated in the structures.
Some known approaches consist in proceeding with a reference measurement of the structure in the healthy state in order to observe a difference with a subsequent state to reveal the presence of the defect. In order to reliabilize this operation, various signal processing techniques exist, in particular for neutralizing the influence of temperature, but none is truly effective. In all cases, the interpretation of the signals remains very difficult.
The various aspects of the invention mitigate these drawbacks, at least partly.